Membrane separation processes which use a plurality of semi-permeable membranes in series for the separation of one or more, more permeable components from one or more, less permeable components in a feed stream are known in the art. A state of the art example in commercial use today is the "series type two unit separation cell" process as shown in FIG. 1 which recovers both the permeable and non-permeable components at desired purity while using an optimum combination of power and membrane area vis-a-vis any comparable process. A disadvantage of FIG. 1's process is that it usually has mixing of streams with different compositions and its attendant energy loss.
U.S. patent application Ser. No. 07/933,152 by Xu introduces the concept of a compressed permeate sweep which can significantly improve the energy efficiency of membrane separation processes at the expense of increased membrane area. One particular application of Xu to FIG. 1 results in the process as shown in FIG. 2. When compared to FIG. 1's process for a typical separation, FIG. 2 was 24% more energy efficient at the expense of a 58% increase in membrane area.
U.S. patent application Ser. No. 08/090,376, also by Xu, teaches a membrane separation process as shown in FIG. 3 which is specifically designed for low pressure feed applications. More specifically, FIG. 3 is suitable for applications where the pressure of the feed stream is lower than the higher pressure side of the membrane separation module. When compared to FIG. 1's process for a typical separation, FIG. 3 was 4% more energy efficient and required 5% less membrane area.
Laguntsov et al in the Journal of Membrane Science (v 67, pp 15-28, 1992) teach a membrane separation process as shown in FIG. 4 which is specifically designed for high pressure feed applications. More specifically, FIG. 3 is suitable for applications where the pressure of the feed stream is equal to the pressure of the higher pressure side of the membrane separation module. In theory, FIG. 4 can avoid energy loss for binary feed mixtures at certain combinations of feed composition, membrane selectivity and product purities. At other combinations, however, FIG. 4 is unable to avoid energy loss due to the mixing of the feed stream with the stream that is recycled to the feed stream. In such cases, other schemes have to be developed to avoid such mixing losses. It is an object of the present invention to develop such a scheme. More specifically, it is an object of the present invention to recover either or both of the permeable and non-permeable components at desired purity while using an optimum combination of power and membrane area vis-a-vis any comparable process.